1. Field of the Invention
This invention relates to fluids used to treat oil and gas wells, and more particularly, to low residue well treatment fluids, and to methods of making these fluids and using these fluids to treat oil wells.
2. Description of the Background
Gelled aqueous fluids have been developed and used extensively in the treatment and operation of subterranean formations. Such fluids have been applied in a variety of operations, including well completion, production stimulation, fluid loss control, and water production control.
For example, gelled fluids have been used to increase productivity and overcome well bore damage in subterranean formations through a process known as hydraulic fracturing. In the hydraulic fracturing process, a viscous fracturing fluid is pumped into the well bore at a pressure sufficient to create and extend fractures. A propping agent is usually suspended in the fracturing fluid to be deposited in the induced fractures and hold the fractures open after the pressure is released and the fracturing fluid flows back to the well. After the formation is fractured and the propping agent is in place, the fracturing fluid is usually broken by oxidizer breakers. These oxidizer breakers reduce the viscosity of the fracturing fluid so that the fluid is easily removed from the formation and, as a result, the production from the formation is not inhibited by the viscous fracturing fluid. The propping agent-filled fractures generated in the process serve as high conductive channels, thereby allowing produced fluids, such as hydrocarbon, to easily flow back to the well bore.
The most important component of the fracturing treatment process is the fracturing fluid. Different types of fracturing fluids have been developed. Polymer based aqueous fracturing fluids are the most widely used fracturing fluids used in well stimulation.
The main components of conventional polymer aqueous fracturing fluids are water, a water-soluble polymer, a crosslinking agent, a breaker, a buffer, and surfactants. Polymers commonly used in fracturing fluids include guar gum and chemically modified guar gum derivatives such as hydroxypropyl guar (HPG), carboxymethylhydroxypropyl guar (CMHPG) and carboxymethyl guar (CMG). Guar gum is the most widely used polymer in fracturing fluids because of its low cost. However, when the guar gum in guar gum polymer based fracturing fluid is broken by oxidizers, insoluble residue is generated. This residue can cause blockage of the generated conductive channels and a decrease in the production rate of the formation.
Although guar gum derivatives have previously been considered to be xe2x80x9ccleaner polymersxe2x80x9d than natural guar gum (i.e., generating less residue when broken by breakers), nonetheless, some test results indicate that the residue generated and the resulting damage is about the same for guar gum and guar gum derivatives.
A number of alternative fracturing fluids have been developed in an effort to allow fracturing of a well with less formation damage. For example, U.S. Pat. No. 5,681,796 to Nimerick describes a fracturing fluid system with a reduced polymer concentration and a buffering system. Although the polymer loading may be reduced, the insoluble residue generated from the broken polymer still causes blockage of the generated conductive channels and a decrease in the formation production rate.
Another type of non-polymer fracturing fluid, i.e., a viscoelastic surfactant system, is described in U.S. Pat. No. 5,551,516 to Norman et al. The fluid of this system is comprised of a water-soluble salt, various quaternary ammonium halide salt surfactants as thickening agents, and an organic stabilizing agent such as sodium salicylate. This solid free surfactant system is described as having thermal stability, adequate viscosity and proppant transport capability. The viscosity of this fluid is a result of the rod-like micelle structure rather than the presence of a polymer. The absence of polymer residue presents fewer problems for fracture clean-up as compared to polymer based fracturing fluids.
One disadvantage associated with such viscoelastic systems relates to their ineffectiveness when applied to high permeability formations. Because there is no polymer in the viscoelastic surfactant fracturing fluids, no filter cake is formed once this fluid is pumped into the formation. As a result, the fluid loss to the formation may be high. In addition, because the fluid may penetrate into the formation, there is an increased risk of formation damage and decreased fracture length. In addition, the cost of surfactant based fracturing fluids is high.
Thus, there is a need for a well treatment fluid that combines the benefits of both polymer systems and viscoelastic surfactant systems, i.e., a well treatment fluid which generates minimal residue, while at the same time minimizing fluid leak off, for example, during fracturing operations.
The present invention overcomes problems and disadvantages associated with current fluid compositions, and provides well treatment fluids using natural and derivatized polysaccharides modified with hydrophilic groups to increase polymer solubility. The modified polysaccharides in the preferred fluids of the invention generate minimal or no residue upon being broken, thereby causing less damage to the formation and proppant bed. At the same time, preferred fluids form a filter cake upon contact with the formation to prevent fluid leak off. Well treatment fluids of the invention are particularly useful in fracturing operations.
In preferred embodiments of the invention, the well treatment fluid is broken in a controlled manner by a gel breaker, which is preferably a delayed gel breaker, in the fluid. Because the broken polymer fragments of the modified polysaccharide are easy to dissolve, little or no polymer residue is generated and left in the treated subterranean zones when the fluid is broken. The present invention avoids the problems seen with the polymer thickeners used in conventional fluids, which form insoluble residues that may damage the formation, at either regular or reduced gelling agent concentrations.
Accordingly, one embodiment of the invention is directed to a well treatment fluid comprising: an aqueous solvent; a gelling agent comprising one or more modified polysaccharides, the modified polysaccharides having hydrophilic groups; and a crosslinking composition. Preferably, the fluid further comprises a gel breaker, a buffer and proppant.
The invention also includes methods to treat and/or fracture a subterranean formation. One such method comprises the step of injecting a well treatment fluid according to the invention into the formation. Preferably, the fluid is injected at a sufficient pressure to generate a plurality of fractures in the formation. Preferably, the fluid carries a proppant into the plurality of fractures and deposits the proppant in the fractures. Preferably, the fluid further includes a gel breaker, and the method further comprises allowing the gel breaker to break the well treatment fluid down to a thin fluid, and allowing the thin fluid to flow back through the well bore to the surface.
Still another embodiment is directed to a method of preparing a well treatment fluid comprising the steps of: providing a plurality of components comprising an aqueous solvent, a gelling agent, and a crosslinking composition; and combining the plurality of components to form a well treatment fluid having a viscosity of above about 50 centipoise at 100 secxe2x88x921. The gelling agent comprises one or more modified polysaccharides having hydrophilic groups. The method may optionally comprise adding a buffer, a gel breaker and/or proppant to the fluid.
Other objects and advantages of the invention are set forth in part in the description which follows, and, in part, will be obvious from this description, or may be learned from the practice of the invention.